It is conventionally known that the time (pulse wave propagation time) required for a pulse wave to propagate between two points in a living body or the pulse wave propagation velocity obtained by dividing the blood vessel length between the two points by the pulse wave propagation time has a correlation with the blood pressure. A method of continuously monitoring the blood pressure by continuously measuring, e.g., the pulse wave propagation time by using this relationship is proposed (e.g., Japanese Patent Laid-Open No. 10-66681).
To measure the pulse wave propagation time, however, pulse waves must be measured in different locations, so the measurement requires a long time. Also, it is sometimes difficult to attach sensors or cuffs for measuring pulse waves to two locations. As described in Japanese Patent Laid-Open No. 10-66681, therefore, an electrocardiogram (ECG) is sometimes used instead of one pulse wave. In this case, time difference between the R wave appearance point of the ECG and the feature point of a pulse wave measured at a fingertip is used as the pulse wave propagation time.
In an operating room or ICU, an ECG and pulse wave (at one location) are normally measured at all times. Accordingly, when the ECG is used as one pulse wave, the pulse wave propagation time can be measured without adding any apparatus, so the method is advantageous in cost and operability.
Unfortunately, the use of an ECG has a problem of the measurement accuracy. That is, an ECG is a signal which represents not a pulse wave but the electrical state change of the heart. There is time difference (preelection period) between the timing at which the electrical state change occurs and the timing at which the heart actually contracts to generate a pulse wave. This preelection period has influence on the pulse wave propagation time to be measured.
If the preelection period is constant, correction is easy. However, the preelection period changes from one person to another, and can change occasionally even in the same person. This presently makes the preejection period hard to correct. Accordingly, it is difficult to obtain results more accurate than when blood pressure is calculated from the propagation time obtained from two pulse waves.